Though often fiercely partisan, Americans have no great love for political parties as such. Ever since James Madison wrote his mistrust of "factions" into the Constitution, parties and their "bosses" have been repeatedly attacked as privileged insiders bent on thwarting or twisting democratic processes.

Madison's plan worked, partially. With 50 state governments and with a federal government divided between a bicameral legislative branch and a president, the United States produces parties that are relatively unstructured and ideologically amorphous - and generally only two of them. Parliamentary systems encourage multiple disciplined parties, representing more, and more distinct, interests and sentiments.

The other side of the story is that American parties still provided valuable public services, including the facilitation of collective action by like-minded, or at least compatible, citizens; continuityand responsibility in ideology; and, last but not least, the vetting of aspirants for public office.

Well-functioning parties are political gatekeepers, necessary to representative democracy but antithetical to the utopian alternative, direct democracy.

A belief in direct democracy, apparently, is behind the attack by Sen. Bernie Sanders, I-Vermont, and other Democratic "progressives" on the institution of "superdelegates" - elected officials and other insiders who get automatic votes at the Democratic Party's convention, much to their fellow insider Hillary Clinton's advantage, the critics allege.

"Rigged system!" Sanders cries. It's hard to separate Sanders's proclaimed principles from self-interest and sour grapes, especially because when he's not denouncing the existence of superdelegates, he's desperately trying to get their votes. Until last year, Sanders thought himself too pure a progressive to actually join the party he now presumes to lead and to lecture.

But to the extent he is making a good-faith claim - that it's undemocratic to allocate a critical mass of convention votes to 700-plus elected officials and other party "regulars," rather than let primary voters, non-Democrats included, pick new delegates every four years - it's a simplistic one.

Parties are entitled to think about continuity and electability, without which, obviously, they can never achieve their policy goals. Hence, they're entitled to favor loyalists, like the superdelegates, and known quantities, like Clinton - for all her flaws - over interlopers, like Sanders.

The tension between the internal discipline necessary to the efficient functioning of left-wing political parties, on the one hand, and these parties' egalitarian principles, on the other, is a commonplace of political analysis: German sociologist Robert Michels called it the "iron law of oligarchy" way back in 1911, after making a careful study of Germany's Social Democratic Party - the original democratic socialists.

If Sandersistas ever took over the Democratic Party, as they might yet do, they too would eventually mutate from dewy-eyed outsiders to system-rigging insiders. Heck, Clinton got her start organizing Texas's African Americans and Latinos for the left-wing insurgent Democratic presidential nominee George McGovern in 1972.

The McGovern campaign grew out of the mother of all Democratic internal- democracy psychodramas. After a convention dominated by party regulars picked Vice President Hubert Humphrey for president in 1968, Democrats promised the next nomination race would be open to '68's outsiders. McGovern - who helped draft the new procedures - went on to capture the nomination, then lead Democrats to a landslide defeat.

Jimmy Carter, another rules-enabled outsider, eked out a win in 1976, but his reelection failure in 1980 persuade Democrats to reempower moderate party veterans, resulting, ultimately, in the superdelegate rule.

Nevertheless, some on the Democratic left appear bent on abolishing the superdelegate rule to appease the Sandersistas and make a statement about democracy.

As Sanders himself said with unintended irony Tuesday, "Defying history is what this campaign has been about."

Clinton got more primary votes and non-superdelegates than Sanders did anyway; thus, as many election analysts have noted, she probably would have won sans superdelegates.

Still, the latter served as a fail-safe, protecting the party against a hostile outside takeover in the event that Sanders denied Clinton a majority of pledged delegates.

If only the Republicans had such a circuit breaker! Instead, they were left at the mercy of an untameable intruder, Donald Trump, and the large, motivated minority of primary voters he inflamed by attacking the GOP and its leaders - as well as by vilifying various minority groups and repeatedly violating basic behavioral norms.

Laugh at the Republicans' comeuppance if you want; heaven knows it's richly deserved. But now the entire country is at risk of a Trump inauguration in 2017.

Decry, if you must, the party "duopoly" that has presided over, not resolved, the country's recent troubles - but also kept us on an even keel in now-forgotten better times.

When Democrats and Republicans have passed through this crucible of disruption and realignment, we will still need them, or some new, improved version, to frame issues, channel political participation, select candidates and, one hopes, forge consensus.

No party can perform any of those functions without the power to differentiate between "one of us" and everyone else.